Single photon detection circuits are useful for a number of applications such as LiDAR, photoluminescence or fluorescence detection, and quantum information. One approach to detect single photons uses what is referred to as a single photon avalanche diode (SPAD), also sometimes called a Geiger-mode avalanche photodiode (GmAPD). In a SPAD, a photodiode is biased beyond breakdown and a single photon is all that is necessary to trigger the breakdown of the diode and create a large current that is easily detected.
SPAD technology has the advantage of being semiconductor based so that it is compact, cheap, and reliable. The best SPAD devices, by far, are made of silicon, however, silicon SPADs have significant wavelength limitations. Due to the band edge of silicon, silicon SPADs only function at wavelengths <1000 nm, and also often are not very efficient at detection of the longer wavelengths within this range (in the near infrared).
To make a SPAD that is usable at longer wavelengths, indium gallium arsenide (InGaAs)/indium phosphide (InP) devices have been used instead of silicon. However, InGaAs/InP SPADs are dramatically inferior to Si SPADs, with much higher dark count rates (output current when no photo-input exists), they do not operate at room temperature, and they are susceptible to after-pulsing while still having relatively low efficiencies of around 10%-20%. in additional, InGaAs/InP devices are significantly more expensive to fabricate than silicon devices.
Other single photon detection approaches include photomultiplier tubes and superconducting nanowire single-photon detectors (SNSPD). But photomultiplier tubes are bulky devices requiring very high voltages to operate. And while SNSPD offer the best performance of any single photon detector, they also require cooling below 10° K. Attempts also have been made to directly fabricate SPADs using germanium, but those devices have extremely high dark count rates, making them unsuitable for most applications.